Environmental Engineering Reference
In-Depth Information
process converting solar energy in vegetable material is taken into account
(see
Sect. 1.3
). Recent studies have evidenced that the global potential for biomass
energy production could substitute not more than a few percent of current fossil
fuel usage [
48
].
Hydrogen is a fuel much different from those above considered, due to its
nature of carbon-free fuel, which implies obvious benefits in terms of exhaust
emissions. However when combustion is used as an oxidation process to
provide energy, NO
x
emissions are unavoidable, together with traces of
hydrocarbons and CO deriving from engine lubricant. A part from the absence
of carbonaceous emissions, the great interest towards hydrogen as a transpor-
tation fuel is based on its thermal properties. Its specific energy is about three
times higher with respect to both conventional liquid fuels (gasoline and Diesel
fuel) and natural gas; however, its volumetric energy density at atmospheric
pressure is more than three times lower than natural gas. This implies that if
hydrogen has to be used in automobile fuel tanks, it needs to be compressed at
very high pressure (at least 700 bar) to ensure a driving range per refill
comparable to that of conventional vehicles, or stored as a liquid in cryogenic
cylinders (see
Sect. 2.3
). Regarding its utilization in internal combustion
engines for road vehicles, hydrogen presents some beneficial characteristics,
such as very higher octane number and wider flammability than gasoline. While
low ignition energy and high flame speed can cause self-ignition phenomena or
flame flash-back during mixture preparation, the wider ignition range permits
very lean air/fuel mixtures to be burnt, giving a way of controlling the NO
x
emissions. Recent developments of BMW have led to the seven series H
2
prototype [
49
], with liquid hydrogen on board and an electronic fuel mixture
system, able to exactly regulate the hydrogen intake. The combustion on this
prototype is generally lean, with reduced NO
x
emissions, while the insulation
system of the cryogenic tank ensures loss-free storage of hydrogen for about
3 days with a refilling time of about three minutes.
If the aspect of vehicle exhaust emissions is considered as a priority, the above
considerations show that hydrogen is the most suitable fuel for the technology
currently adopted to power most of today vehicles (i.e. internal combustion
engines). However, other critical issues related to usage of this energy carrier, in
particular production and storage methods, besides the need of dedicated infra-
structures (see
Chap. 2
), suggest to evaluate the convenience of its utilization in a
wider context, taking into account atmospheric emissions at global level, and
estimations of well-to-wheels efficiencies.
1.3 Climate Change and Carbon-Free Fuel Chance
The greenhouse effect is a natural process which helps to regulate the earth's
temperature, avoiding planet freezing. About half of the visible radiation coming
from sun pass freely through the atmosphere and is transformed in heat when
